Tips & Tricks for Teaching Dichotomous Keys in IBDP Biology & ESS
Dichotomous keys are one of my favourite parts of teaching about the diversity of life as they require creativity, organization, attention to detail and application of knowledge. In case you are unfamiliar, dichotomous keys are tools used to identify unknown species using a series of simple two-choice questions which can be found in either a flow chart or table form (see the examples below). The goal of using or creating a dichotomous key is to identify and unknown group or taxa of life; they are particularly useful in fieldwork and biodiversity studies.
You can find this virus dichotomous key for free in my Teachers Pay Teachers shop, just click the image to make your way there! Tables like this one are my preferred way to make dichotomous keys due to their clearly communicated format and the ease provided by using reference numbers for each step.
This flow chart style dichotomous key (the other acceptable format for a key) is from BioNinja, an excellent tool for all IBDP Biology students and teachers, they are currently updating the website for the new syllabus, so check it out!
Introducing Dichotomous Keys
When introducing dichotomous keys to my students I always take the time to work through some
very simple ones as we start, this one about a snake, chicken, duck and lizard is my favourite as it
requires no prior knowledge. We then take some time to practice using a more complex key for Toads of Algonquin park as a class and finally students complete the virus key (above) individually, which we then take up after about 15 minutes of work time.
Some key things to discuss with your class at this stage include:
Using readily observable traits (ex. if the organism isn't swimming when you see it is "swims" a good trait to use in your key?)
Avoid subjective descriptors like "big" or "small", try to quantify or at least compare, for example "tail is at least twice as long as head" is much better than "tail is long"
Determine the background knowledge of your audience, if necessary provide some background information, such as basic anatomy of the taxa in question to help in your descriptions (for example, if you don't know what a capsid or envelope is the virus key is hard to use)
Making Dichotomous Keys
After we have used keys made by others it is time to practice making a dichotomous key and my favourite way to do this is with shoes! All of the class joins together in front of the board and everyone takes off one of their shoes. They then work together to make a dichotomous key for the shoes, using traits like brand, colour, size, shoe type etc. (if you have a very large class I recommend splitting into two groups). As students shoes are incorporated into the key they can take their shoes back and put them on again. This works really well because it's funny, no prior knowledge is required and by working as a group it is easy to take advantage of teachable moments to clarify any misunderstandings.
Here is the dichotomous key made by my ESS students just a few weeks ago when we learned this concept as part of our Topic 2 unit. Teacher tip: wear some fun and silly shoes when you're planning to do this activity and you'll get your shoes back quickly!
Once students have practiced as a class it is time to assign their dichotomous key assignment, which looks a little bit different depending on which course they are in. Here is an example of a dichotomous key assignment for beetles that I used for the old (last exam May 2024) IBDP Biology syllabus, rubric included.
Assessing Dichotomous Keys in IBDP Biology & ESS
The specific requirements for Dichotomous keys are different in each course, so let me clarify what you need to do depending if you are teaching the new IBDP Biology, the old IBDP Biology or ESS. Whichever course I am working in I create rubrics using the IA descriptors provided by the IBO, you can see an example of this in the Beetle Dichotomous Key above.
Old IBDP Biology (2016 syllabus)
This skill is part of Subtopic 5.3 Classification of Biodiversity and can be found on page 70 of the IBDP Biology subject guide. Since Topic 5 is an SL subtopic every student needs to make a dichotomous key. The guide does not specify a species type or a number so I have used beetles and Hawaiian honeycreepers for this assignment, with up to 16 individuals to identify. Since I teach this topic quite early in the course it is a useful knowledge and communication assessment that can help me to determine if students are in the right IBDP science course.
New IBDP Biology (2025 syllabus )
In the new IBDP Biology syllabus this skill is part of Theme A: Unity and Diversity, Level 3 Organisms and it is only for HL students, it can be found on page 44 of the new guide. This one specifies that it must be regarding local plant or animal species, so this year my students went out into the campus grounds to choose six plant species to identify. Since students were choosing the species themselves they needed to take pictures and use an App like NatureID to identify the species they had decided to include in their key. I plan to work on developing this assignment further and making a resource for my TPT shop, when it's done I will link it here.
IBDP ESS (2017 syllabus)
In the current ESS syllabus making dichotomous keys is part of Subtopic 2.5 Ecological Investigations and can be found on page 39. This guide is unique in that it specifies the number of species with only eight to be used. For this reason I provided students with an assignment based on the Hawaiian Honeycreeper and had them make a key for any eight of the included birds. Having more choices helps to ensure that students work is original and allowing for choice always increases student engagement.
Dichotomous keys easily take an entire lesson and depending if you are giving your students work time for the assignment they can certainly take two more more. I recommend you have your students check each other's keys before handing them in, peer feedback is a particularly useful strategy for a task such as this one.
I hope you found this post helpful and that you try the shoe activity with your class! It's one of my favourites and requires no prep!
Thanks for reading teachers, travelers and curious souls of all kinds.
The Roaming Scientist
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